Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation. They may also take the form of implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, which electrically couple the pacemaker to the heart.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Further, pacing systems are known which pace in multiple sites. For example, biventricular pacing paces in both ventricles and biatrial pacing paces in both atria. Hence, it is possible, that a heart may be paced in all four of its chambers.
A popular mode of operation for dual-chamber pacemakers is the DDD mode. Specifically, DDD systems provide atrial pacing during atrial bradycardia, ventricle pacing during ventricular bradycardia, and atrial and ventricular pacing during combined atrial and ventricular bradycardia or heart block also known as AV block. In addition, DDDR systems monitor patient activity levels for controlling pacing rate to more closely approximate the normal response of the heart to exercise, or other physiological activity demanding a faster heart rate.
Recent studies have indicated that ventricular pacing in the setting of intact AV nodal conduction has an adverse impact compared to permitting intrinsic ventricular contractions. Hence, pacing therapies have been advanced which encourage intrinsic ventricular activity.
One such system employs an auto intrinsic conduction search (AICS) wherein the pacemaker utilizes two AV intervals. The first AV interval is a programmable base AV interval to support ventricular demand pacing. It may be, for example, on the order of two hundred (200) milliseconds. The second AV interval is an extended AV interval which may be thought of as comprising the base AV interval with an AV interval extension added to its end. The AV interval extension may be on the order of one hundred (100) milliseconds, for example. Hence, in this example, the extended AV interval would total three hundred (300) milliseconds.
The AICS may be implemented as follows. The device paces in a demand mode with the base AV interval. After a timed interval of five minutes, for example, the device extends the AV interval to the extended AV interval for one cycle to encourage intrinsic ventricular activity. The device does not reset to the shorter base AV interval until a ventricular pacing pulse is administered.
Other methods to promote or encourage intrinsic conduction through AV interval extension have been advanced. Hence, the term AV hysteresis is meant to encompass all such methods involving AV interval extension to encourage intrinsic heart activity.
Unfortunately, some problems can arise as a result of an AV interval extension during the implementation of AV hysteresis. During the extended AV interval, the atrium may have recovered on a physiologic basis to allow retrograde conduction to occur and the initiation of a pacemaker mediated tachycardia (PMT). Repeated stimulation at a high rate can there after be sustained by heart tissue retrograde conduction and by pacemaker anterograde conduction.
Methods for preventing PMT are well known in the art. One such known method involves the use of programmable post-ventricular atrial refractory periods (PVARP), where the PVARP is programmed to be longer than the retrograde conduction interval. Unfortunately, standard PVARP intervals in the setting of AV interval extension during AV hysteresis have not always been successful in avoiding the generation of a PMT. Hence, there remains room for improving AV hysteresis and preventing PMT's during their use.
In addition to potential PMT's, other arrhythmic heart rhythms may result from AV hysteresis. One such arrhythmic heart rhythm is a repetitive non-reentrant ventriculo-atrial synchronous (RNRVAS) rhythm. The RNRVAS rhythm is fully described, for example, in U.S. Pat. No. 6,498,949 B2, which patent is incorporated herein in its entirety.
Another arrhythmic heart rhythm that may result from AV hysteresis is AV nodal reentrant tachycardia (AVNRT). In this case, a P wave coinciding with an extended PVARP is not tracked, however, if it is conducted with a long PR interval, it may return in a retrograde manner through a second pathway within the AV node or peri-AV nodal tissues allowing for sustained intrinsic supraventricular reentrant tachycardia.
It would be most desirable if an implantable cardiac stimulation device could both avoid PMT's and deal with other arrhythmic cardiac rhythms during or as a result of AV hysteresis. The present invention addresses this and other issues.